Assert RequestCleanup ran only once

Prevent the RequestCleanup.run() to be called more
than once.
Helps preventing bugs in the request context cleanup
process and allow making changes to the Git/HTTP and
Git/SSH protocols validating the correct request context
management.

Change-Id: I432d36f591ee9015856ac18ad27e98dcdca8e465

java/com/google/gerrit/server/RequestCleanup.java

@@ -31,9 +31,7 @@ public class RequestCleanup implements Runnable {
   /** Register a task to be completed after the request ends. */
   public void add(Runnable task) {
     synchronized (cleanup) {
-      if (ran) {
+      assertNotRan();
-        throw new IllegalStateException("Request has already been cleaned up");
-      }
       cleanup.add(task);
     }
   }
@@ -41,6 +39,7 @@ public class RequestCleanup implements Runnable {
   @Override
   public void run() {
     synchronized (cleanup) {
+      assertNotRan();
       ran = true;
       for (Iterator<Runnable> i = cleanup.iterator(); i.hasNext(); ) {
         try {
@@ -52,4 +51,10 @@ public class RequestCleanup implements Runnable {
       }
     }
   }
+
+  private void assertNotRan() {
+    if (ran) {
+      throw new IllegalStateException("Request has already been cleaned up");
+    }
+  }
 }